Explosives currently being used in rock blasting situations are generally high shock energy explosives in which all of the explosive energy and the attendant high-pressure gases are generated more or less instantaneously. A typical example of such an explosive which is currently used is ANFO which is a mixture of ammonium nitrate (AN) and vegetable and mineral oils with flash point greater than 140.degree. F., typically diesel oil No.2 (FO). The use of ANFO explosives in many blasting situations results in a number of disadvantages which include the following:
(i) The explosive releases energy in two main forms--shock, and heave energy. At detonation there is a sudden increase of pressure that displaces the blasthole wall, generating a strain, or shock, wave that produces cracks in the rock. The energy in this wave is the shock energy. After the shock wave has propagated through the rock, the hot pressurised gas which is left in the blasthole is able to extend the cracks as well as to heave the burden. The gas has an energy content called the heave energy. Before blasting, rock generally contains sufficient fractures that can be propagated by the heave energy alone. Thus the shock energy serves little or no useful purpose in fractured rock. For ANFO 94/6 (94% Ammonium Nitrate/6% Fuel Oil), the total energy theoretically available is 3727 J/g, which comprises 1241 J/g shock energy, 2255 J/g heave energy and 231 J/g of residual energy, where the residual energy is the internal energy of the gas itself and cannot be utilised. PA1 (ii) Due to the high shock energy generated by the explosion a greater proportion of fine rock particles (fines) are produced by the shock wave crushing the rock located in close proximity to the borehole more than is desirable or is required, such as for example, for use in further processing steps. PA1 (iii) Minerals, or other materials of economic value, such as for example, diamonds which are to be extracted from the rock are sometimes damaged by the crushing of diamond bearing rock caused by the shock wave, particularly in locations close to the blasthole.
It is thought that the development of a low shock energy explosive in which more of the energy of the explosive is generated as heave energy and less as shock energy, and where the energy is more gradually released, may alleviate at least some of the problems associated with the use of conventional high shock energy explosives. Therefore, it is an aim of the present invention to provide a modified explosive, particularly a modified high shock energy explosive which is useful in blasting, in which the production of shock energy is reduced somewhat when compared to conventional blasting explosives.
Previous attempts to produce a LSEE involved dilution of the explosive mixture to produce a lower bulk energy for a given mass of explosive mixture. In general, previous attempts have resulted in low shock, low bulk energy explosives which necessitates the drilling of more blastholes. For example, ANFORGAN is a known form of LSEE that consists of a mixture of ANFO and sawdust, typically in the ratio of about 2:1. The sawdust acts as a diluent for the ANFO which reduces the density of the explosive mixture. It is well known that the shock energy of an explosive decreases as its density decreases. The problem with reducing the density of the explosive is that in a blasthole the amount of explosive is limited by the volume of the hole. A low density explosive will not have as much mass in a given volume as a high density explosive. Since the effects of the explosive are related to the amount of explosive in the hole, a low density explosive will not break the rock as effectively as a high density explosive. It is an object of the present invention to lower the shock energy but to keep the total energy at a level comparable to a conventional explosive, such as ANFO.